This invention relates to methods of insulating surfaces and enclosures with plastic structures containing a carbon black of certain configuration and type in a quantity sufficient to reduce the thermal conductivity of the structure.
Carbon black is known as an infrared radiation absorber and reflector, and has been commercially employed in insulative foam structures to reduce thermal conductivity. Extensive teachings of that use are seen in U.S. Pat. No. 4,795,763 and WO 90/06339, both of which are incorporated by reference herein. They disclose several types of carbon black, and further disclose useful particle size ranges and loading (i.e. weight) levels.
Problems with employing carbon black in insulative foam structures include processing problems and product quality problems.
One processing problem is the considerable difficulty of homogeneous dispersing or mixing the carbon black in a polymer melt or polymer gel of the melt with a blowing agent. Carbon black previously employed in insulative structures, furnace black, is commercially supplied in clusters of 2-90 particles due to thermal fusion during its manufacture. These clusters are practically impossible to break up during dispersion in the polymer melt or gel. Further, the clusters themselves are difficult to disperse in a polymer melt or gel due to electrostatic attraction between the clusters themselves. The dispersion problems may require expensive pre-compounding of the carbon black in the base polymer prior to manufacturing the foam structure. Pre-compounding is very expensive, and the carbon black still may not disperse sufficiently to achieve maximum reduction in thermal conductivity in the insulating foam structure or avoid processing or product problems. Further, the high loading levels of carbon black required to attain substantial reduction in thermal conductivity, typically 4-10 percent by weight of the foam structure, may further exacerbate the processing problems described above.
Another processing problem is the relatively large surface area and porosity of the previously-employed carbon black may attract and thus inhibit the activity of other additives during manufacture of the foam structures. This inhibition may result in a reduced additive activity, higher additive loading requirements, or switching to less desirable additives.
Another processing problem of the previously-employed carbon black is its tendency to cling or adhere to surfaces, process and otherwise. This surface cling results in housekeeping, cleanliness, and personal hygiene problems.
Another processing problem is an increase in process pressure drop observed upon addition of the previously-employed carbon black to a gel of polymer melt and blowing agent. The increased pressure drop may result in poorer skin quality, increased power consumption, increased equipment pressure duty, and increased die pressure.
Another processing problem is that high loading levels of the previously-employed carbon black can also create skin quality problems in extruded foam structures.
Another problem with using the previously-employed carbon black in foam structures is printability. The dark, black color of such foam structures makes printing on the foam structure difficult.
Another problem with foam structures employing the previously-employed carbon black, or, for that matter, foam structures without carbon black, is that of flame retardancy. Fire retardant additives are typically employed to enhance flame retardancy to desirable levels.
It would be desirable to find a carbon black which more readily disperses in a polymer melt. It would be further desirable to find a carbon black which enhances the dispersion in the pre-compounding step or possibly eliminates the need for it. It would be further desirable to find a carbon black which does not induce substantial process pressure drop or poor skin quality. It would further be desirable to find a carbon black which might possibly enhance the insulating capability of the foam structure such that the loading of carbon black required to achieve a given level of insulation might be reduced. It would further be desirable to have a foam structure that could more easily be printed on. It would be further desirable to find a carbon black which provides a foam structure with a level of flame retardancy not observed in the prior art.